[ruby/irb] Reduce internal operations' exposure to benchmarking

(https://github.com/ruby/irb/pull/618)

* Test last value is assigned with measure enabled

* Remove unnecessary `result` variable

`Context#evaluate` always assigns the result of the evaluation to `_` so
we don't need to do it in `Irb#eval_input`.

* Move benchmarking logic into `Context#evaluate`

Current location of the benchmarking logic is too high up and includes
operations like command loading and argument transformation, which should
be excluded. So this commit moves it into `Context#evaluate` to reduce the
noise.

We don't move it further down to `Workspace#evaluate` because `Context`
is an argument of the measure block, which is not available in `Workspace`.

lib/irb.rb

@@ -558,26 +558,10 @@ module IRB
       @scanner.each_top_level_statement do |line, line_no|
         signal_status(:IN_EVAL) do
           begin
-            if IRB.conf[:MEASURE] && IRB.conf[:MEASURE_CALLBACKS].empty?
-              IRB.set_measure_callback
-            end
             # Assignment expression check should be done before evaluate_line to handle code like `a /2#/ if false; a = 1`
             is_assignment = assignment_expression?(line)
-            if IRB.conf[:MEASURE] && !IRB.conf[:MEASURE_CALLBACKS].empty?
-              result = nil
-              last_proc = proc{ result = evaluate_line(line, line_no) }
-              IRB.conf[:MEASURE_CALLBACKS].inject(last_proc) { |chain, item|
-                _name, callback, arg = item
-                proc {
-                  callback.(@context, line, line_no, arg) do
-                    chain.call
-                  end
-                }
-              }.call
-              @context.set_last_value(result)
-            else
-              evaluate_line(line, line_no)
-            end
+            evaluate_line(line, line_no)
+
             if @context.echo?
               if is_assignment
                 if @context.echo_on_assignment?

lib/irb/context.rb

@@ -475,7 +475,29 @@ module IRB
 
     def evaluate(line, line_no) # :nodoc:
       @line_no = line_no
-      set_last_value(@workspace.evaluate(line, irb_path, line_no))
+      result = nil
+
+      if IRB.conf[:MEASURE] && IRB.conf[:MEASURE_CALLBACKS].empty?
+        IRB.set_measure_callback
+      end
+
+      if IRB.conf[:MEASURE] && !IRB.conf[:MEASURE_CALLBACKS].empty?
+        last_proc = proc do
+          result = @workspace.evaluate(line, irb_path, line_no)
+        end
+        IRB.conf[:MEASURE_CALLBACKS].inject(last_proc) do |chain, item|
+          _name, callback, arg = item
+          proc do
+            callback.(self, line, line_no, arg) do
+              chain.call
+            end
+          end
+        end.call
+      else
+        result = @workspace.evaluate(line, irb_path, line_no)
+      end
+
+      set_last_value(result)
     end
 
     def inspect_last_value # :nodoc:

test/irb/test_cmd.rb

@@ -252,6 +252,34 @@ module TestIRB
       assert_empty(c.class_variables)
     end
 
+    def test_measure_keeps_previous_value
+      conf = {
+        PROMPT: {
+          DEFAULT: {
+            PROMPT_I: '> ',
+            PROMPT_S: '> ',
+            PROMPT_C: '> ',
+            PROMPT_N: '> '
+          }
+        },
+        PROMPT_MODE: :DEFAULT,
+        MEASURE: false
+      }
+
+      c = Class.new(Object)
+      out, err = execute_lines(
+        "measure\n",
+        "3\n",
+        "_\n",
+        conf: conf,
+        main: c
+      )
+
+      assert_empty err
+      assert_match(/\ATIME is added\.\n=> nil\nprocessing time: .+\n=> 3\nprocessing time: .+\n=> 3/, out)
+      assert_empty(c.class_variables)
+    end
+
     def test_measure_enabled_by_rc
       conf = {
         PROMPT: {